While age-related cognitive decline and neuronal loss have been well-studied, much less is known regarding the reduced regenerative capacity of the aged brain. Our long-term goal is to elucidate the factors which affect the replicative potential of neural stem cells residing in the adult hippocampus. The broad hypothesis is that age and glucocorticoids interact to induce premature senescence. We base this hypothesis on observations that i) adult neural stem cell proliferation decreases with age, ii) glucocorticoids mediate this effect on hippocampal neurogenesis, iii) glucocorticoids are known to accelerate the aging process, and iv) glucocorticoid exposure shortens telomeres and increases the expression of some tumor suppressor genes in proliferate tissues outside the brain. Based on these observations, the focus of this proposal is to characterize cellular aging in hippocampal adult neural stem cells and determine whether glucocorticoids are risk factors for early onset senescence. The specific aims are as follows: 1) to determine whether adult neural progenitor cells exhibit signs of age-related and glucocorticoid-induced telomere dysfunction in vitro. Across several passages, we will examine telomere lengths, as well as telomerase activity and expression in isolated hippocampal progenitor cells exposed to i) glucocorticoids, ii) glucocorticoid receptor antagonists, iii) cells treated with anti-glucocorticoid transgenes. Preliminary data suggests that these self-renewing cells express glucocorticoid receptors and that an active transcript of telomerase is sensitive to elevated glucocorticoid levels. 2) To examine the role of cyclin dependent kinase (CDK) inhibitors in age-related and glucocorticoid- induced adult neural progenitor cell senescence in vitro. The expression of members of the INK4/CDK/Rb and p21 families of tumor suppressors will be determined as a function of replicative age and glucocorticoid exposure. The participation of identified candidates in the onset of neural stem cell senescence will be determined by the ability RNAi to prevent (or delay) replicative quiescence and SA-p-galactosidase activity. 3) To examine the influence of age and glucocorticoids on the senescence of adult neural progenitor cells in vivo. Using laser capture microscopy, we will corroborate in vitro findings with gene expression profiles and telomere measures in adult-born (as well as perinatally-generated) cells harvested from brain tissue. Since adult neurogenesis may be important for learning and in recovery from traumatic brain injury, an understanding of mechanisms and risk factors for neural stem cell senescence will help to promote neurological health and well-being.